# Re: [Lightning-dev] Liquidity Ads and griefing subtleties

```Unless I'm missing something, we can make option 2 work with CLTV
enforcement as well.  In fact, I think that makes the bookkeeping much
simpler.```
```
Suppose the leased amount is X.  No counters are needed.  All we need
is to ensure the seller's first X sats in the commitment are always
encumbered by the CLTV.  Anything above X can go to a second output
that's unencumbered.  That's it.

Here's an example.  Alice leases 10k sats from Bob and also puts up
10k sats to make the channel balanced.  The initial commitment
transaction looks like this:

Alice: 10k sats
Bob: 10k sats with CLTV

Bob offers a 2k sats HTLC1 to Alice.  The commitment becomes:

Alice: 10k sats
Bob: 8k sats with CLTV
HTLC1: 2k sats

HTLC1 is fulfilled:

Alice: 12k sats
Bob: 8k sats with CLTV

Alice offers a 4k sats HTLC2 to Bob:

Alice: 8k sats
Bob: 8k sats with CLTV
HTLC2: 4k sats

HTLC2 is fulfilled.  Since Bob now has a total balance greater than
10k sats, the excess goes to an unencumbered output:

Alice: 8k sats
Bob: 10k sats with CLTV
Bob: 2k sats

Bob offers a 1k sats HTLC3 to Alice.  The sats always come out of
Bob's unencumbered output first:

Alice: 8k sats
Bob: 10k sats with CLTV
Bob: 1k sats
HTLC3: 1k sats

Bob offers a 3k sats HTLC4 to Alice.  The sats always come out of
Bob's unencumbered output first.  The remaining sats come out of the
encumbered output:

Alice: 8k sats
Bob: 8k sats with CLTV
HTLC3: 1k sats
HTLC4: 3k sats

HTLC3 is fulfilled and HTLC4 is failed.  Bob's total balance will
increase to 11k sats, so the first 10k sats are encumbered and the
last 1k are unencumbered:

Alice: 9k sats
Bob: 10k sats with CLTV
Bob: 1k sats

Alice can never lock up more than 10k sats on Bob's side, since that
was the agreed lease amount.  Bob can still play games with circular
routing or force closing with HTLCs outstanding to unlock some of his
liquidity early, but that is also the case with every other proposal
so far.  Hence the importance of limiting HTLC value in flight.

On Fri, Dec 8, 2023 at 3:48 PM Bastien TEINTURIER <bast...@acinq.fr> wrote:
>
> Hey all,
>
> I'd like to detail a bit more my statement from the last email.
>
> > I personally think it has to be the first option, because the second
> > one exposes the seller to griefing
>
> That is my current conclusion *if we want to provide some kind of lease
> enforcement via CLTV*, and we want to ensure it protects both the buyer
> and the seller equally.
>
> But we can look at it from a different angle: if what people want to
> buy is option 2, then we should find a way to make option 2 work. In
> my opinion, option 2 would be best without any CLTV enforcement of the
> lease, and relying only on incentives (and thus letting the buyer take
> the risk that the seller splices out or force-closes).
>
> But maybe then it doesn't even make sense to have lease durations? Maybe
> we only need to provide a feature to buy X amount of inbound liquidity
> now, and let the seller decide whether they want to keep that liquidity
> available for a long time or move it elsewhere.
>
> Cheers,
> Bastien
>
> Le ven. 8 déc. 2023 à 09:00, Bastien TEINTURIER <bast...@acinq.fr> a écrit :
>>
>> Hey Matt,
>>
>> > The question then is really: do operators want to buy/sell X sats of
>> > inbound flow or Y days of an open channel with an initial inbound
>> > balance of X sats?
>>
>> Agreed, this is what we need to decide. I personally think it has to be
>> the first option, because the second one exposes the seller to griefing
>> by the attack described in my first email, which makes it impossible for
>> the seller to find the right price for that channel because they don't
>> know how much liquidity will actually end up being locked.
>>
>> But that's definitely up for debate if people feel otherwise!
>>
>> Thanks,
>> Bastien
>>
>> Le jeu. 7 déc. 2023 à 22:18, Matt Morehouse <mattmoreho...@gmail.com> a
>> écrit :
>>>
>>> On Mon, Dec 4, 2023 at 9:48 AM Bastien TEINTURIER <bast...@acinq.fr> wrote:
>>> >
>>> > But I've thought about it more, and the following strategy may work:
>>> >
>>> > - store three counters for the seller's funds:
>>> >   - `to_local`: seller's funds that are not leased
>>> >   - `to_local_leased`: seller's funds that are leased
>>> >   - `to_local_leased_owed`: similar to `to_local_leased`, without taking
>>> >     into account pending HTLCs sent by the seller
>>> > - when the seller sends HTLCs:
>>> >   - deduce the HTLC amounts from `to_local_leased` first
>>> >   - when `to_local_leased` reaches `0 sat`, deduce from `to_local`
>>> >   - keep `to_local_leased_owed` unchanged
>>> > - when HTLCs sent by the seller are fulfilled:
>>> >   - deduce the HTLC amounts from `to_local_leased_owed`
>>> > - when HTLCs sent by the seller are failed:
>>> >   - add the corresponding amount to `to_local_leased` first
>>> >   - once `to_local_leased = to_local_leased_owed`, add the remaining
>>> >     amount to `to_local`
>>> > - when creating commitment transactions:
>>> >   - if `to_local_leased` is greater than dust, create a corresponding
>>> >     output with a CLTV matching the lease
>>> >   - if `to_local` is greater than dust, create a corresponding output
>>> >     without any CLTV lease
>>>
>>> Neat idea.  This changes the meaning of liquidity ads slightly -- the
>>> liquidity is only leased for the one-way trip, and any channel balance
>>> that comes back to the seller is not encumbered.  In other words,
>>> instead of the channel itself being leased, only the initial inbound
>>> liquidity is.  Once the cumulative flow to the buyer meets the leased
>>> amount, the seller can reclaim any balance on their side without
>>> penalty.  Of course if there's enough bidirectional flow happening,
>>> the seller may choose to keep the channel open to earn more fees.
>>>
>>> The question then is really: do operators want to buy/sell X sats of
>>> inbound flow or Y days of an open channel with an initial inbound
>>> balance of X sats?
>>>
>>> >
>>> > This computation must occur when sending/receiving `commit_sig`. The
>>> > the `update_fulfill_htlc` updates before the `update_fail_htlc` ones,
>>> > because we need to start by updating `to_local_leased_owed`. I believe
>>> > that works, but it needs more analysis. Please try to break it, and let
>>> > me know what you find!
>>>
>>> On first look, I think this works.  I'll study it closer if we decide
>>> this is the direction we want to go.
>>>
>>> >
>>> > We also need to handle concurrent leases. We want to support the
>>> > following scenario:
>>> >
>>> > - Alice buys 10k sats from Bob for one month
>>> > - 1 week later, the on-chain fees are very low: Alice buys 50k sats
>>> >   from Bob for 6 months
>>> > - 1 more week later, she buys 10k sats from Bob for one week
>>> >
>>> > We thus have three concurrent leases, with overlapping lease durations.
>>> > That's costly for the channel initiator, because we must add three new
>>> > outputs to the commitment transactions. But that part should be ok, the
>>> > seller should price that in its decision to fund the lease or not.
>>> >
>>> > I think we would need to have three `to_local_leased_owed` buckets, one
>>> > per lease. How do we order them, to decide from which bucket we take
>>> > funds first? I think the option that makes the most sense is to order
>>> > them by lease expiry (not by lease start or lease amount, which could
>>> > be unfair to the buyer).
>>> >
>>> > Would that create scenarios where one side can cheat? Are there issues
>>> > with channel reserve because of the increased commit tx weight?
>>> >
>>> > Cheers,
>>> > Bastien
>>> >
>>> > Le sam. 2 déc. 2023 à 03:23, ZmnSCPxj <zmnsc...@protonmail.com> a écrit :
>>> >>
>>> >> Halfway through, I thought "is it not possible to have two Bob-owned
>>> >> outputs that sum up to the total Bob-owned amount, with a CLTV on up to
>>> >> the amount that was purchased and the rest (if above dust limit) without
>>> >> a CLTV?"
>>> >>
>>> >> so e.g. if the purchased amount is 2 units but the total channel
>>> >> capacity is 12 units:
>>> >>
>>> >> * Bob owns 0 units: no Bob outputs
>>> >> * Bob owns 1 unit: Bob has a CLTV-encumbered output of 1 unit
>>> >> * Bob owns 2 units: Bob has a CLTV-encumbered output of 2 units
>>> >> * Bob owns 3 units (assuming 1 unit is above dust limit):  Bob has:
>>> >>   * A CLTV-encumbered output of 2 units
>>> >>   * An ordinary output of 1 unit
>>> >> * etc.
>>> >>
>>> >> This locks up only the agreed-upon amount but lets Bob keep any amount
>>> >> above the rest.
>>> >>
>>> >> Alternately, only allow CLTV-locking if the buyer is not providing its
>>> >> own funds (i.e. pure inbound purchase).
>>> >> This is still effectively my original idea as then any funds Alice wants
>>> >> to add would be in a separate, unencumbered channel.
>>> >>
>>> >> Regards,
>>> >> ZmnSCPxj
>>> >>
>>> >>
>>> >> Sent with Proton Mail secure email.
>>> >>
>>> >> On Friday, December 1st, 2023 at 5:45 PM, Bastien TEINTURIER
>>> >> <bast...@acinq.fr> wrote:
>>> >>
>>> >>
>>> >> > Good morning list,
>>> >> >
>>> >> > I've been thinking a lot about liquidity ads recently, and I want to
>>> >> > highlight some subtleties that should be taken into account in the
>>> >> > protocol design. This is a rather long post, but I believe this is
>>> >> > important to make sure we get it right and strike the right balance
>>> >> > between protocol complexity and incentives design. Strap in, grab a
>>> >> > coffee, and enjoy the ride.
>>> >> >
>>> >> > First of all, it's important to understand exactly what you are buying
>>> >> > when paying for a liquidity ad. There are two dimensions here: an
>>> >> > amount
>>> >> > and a duration. If Alice buys 100 000 sats of inbound liquidity from
>>> >> > Bob
>>> >> > for one month, what exactly does that mean? Obviously, it means that
>>> >> > Bob
>>> >> > will immediately add 100 000 sats (or more) to his side of the channel,
>>> >> > and Alice will pay a fee proportional to that amount *and* that
>>> >> > duration
>>> >> > (because locking funds for one month should cost more than locking
>>> >> > funds
>>> >> > for one day). But is that really the only thing that Alice is buying?
>>> >> >
>>> >> > The current spec proposal adds a CLTV of one month to the seller's
>>> >> > output in the commitment transactions. Without that CLTV, the seller
>>> >> > could accept the trade, and immediately close the channel to reuse the
>>> >> > funds elsewhere. This CLTV protects the buyer from malicious sellers.
>>> >> > But it is actually doing a lot more: what Alice has bought is actually
>>> >> > *any* liquidity that ends up on Bob's side, for a whole month. And the
>>> >> > issue is that this is impossible for Bob to price correctly, and can be
>>> >> > used for liquidity griefing attacks against him.
>>> >> >
>>> >> > Imagine that Alice opens a 1 BTC channel with Bob and asks him to add
>>> >> > 10 000 sats of inbound liquidity for a month. This sounds interesting
>>> >> > for Bob, because Alice is bringing a lot of funds in, so he can expect
>>> >> > payments to flow towards him which will bring him routing fees. And she
>>> >> > isn't asking for a lot of liquidity, so Bob can definitely spare that.
>>> >> > But then Alice sends all the channels funds through Bob to Carol. This
>>> >> > means that Bob now has 1 BTC locked for the whole month, while Alice
>>> >> > only paid for 10 000 sats! He earned some routing fees, but that isn't
>>> >> > enough to make up for the long duration of the lock. If payments keep
>>> >> > flowing in both directions with enough velocity, this is great for both
>>> >> > Bob and Alice. But if the channel is then unused, or Alice just goes
>>> >> > offline, this was a very bad investment for Bob. With splicing, Bob
>>> >> > cannot even know beforehand how much liquidity is potentially at risk,
>>> >> > because Alice may splice funds in after paying for the liquidity ad.
>>> >> >
>>> >> > If Alice pays for a 10 000 sats lease, we only want those 10 000 sats
>>> >> > to be encumbered with a CLTV. But this is actually not enforceable. We
>>> >> > could create a separate output in the commitment transaction with the
>>> >> > leased funds and a CLTV, while keeping the rest of the seller's funds
>>> >> > in
>>> >> > a normal output that doesn't have a CLTV. But then what happens when
>>> >> > HTLCs are relayed and then failed? To which output should we add the
>>> >> > funds back? Any strategy we use here can be exploited either by the
>>> >> > seller to drain the leased funds back to its non-CLTV-locked output,
>>> >> > or by the buyer to keep funds in the CLTV-locked output forever.
>>> >> >
>>> >> > Adding a CLTV thus protects the buyer at the expense of the seller. In
>>> >> > some cases this is ok: if you are a new seller who wants to attract
>>> >> > buyers, you may be willing to take that risk. But in most cases, the
>>> >> > seller is going to be more reputable than the buyer, and has more
>>> >> > incentives to behave correctly than the buyer. When buying liquidity,
>>> >> > you will likely look at the network's topology, and buy from nodes that
>>> >> > are well-connected, or known to be reliable. Or if you are a mobile
>>> >> > wallet user, you'll be buying from your LSPs, who have incentives to
>>> >> > behave honestly to earn fees and attract new users. In those scenarios,
>>> >> > the buyers will very often be new nodes, without any public channels,
>>> >> > which means that the seller has no way of knowing what their incentives
>>> >> > are beforehand. It thus makes more sense to protect the seller than to
>>> >> > protect the buyer. For those use-cases, the simplest solution is to not
>>> >> > add a CLTV at all: the buyer is taking the risk that the seller removes
>>> >> > the liquidity before the end of the lease. But if that happens, they'll
>>> >> > just mourn their lost fees, blacklist that node, and move on. There
>>> >> > will
>>> >> > be a lot more buyers than sellers in that market, so I believe that
>>> >> > this
>>> >> > model makes sense for most large public nodes.
>>> >> >
>>> >> > I think that both use-cases should be supported, so I suggest making
>>> >> > the
>>> >> > CLTV enforcement of the lease optional. It will be up to each seller to
>>> >> > decide whether they are willing to take the risk of locking their funds
>>> >> > to attract buyers or not. Sellers can (should) price that additional
>>> >> > risk in their advertised rates.
>>> >> >
>>> >> > In the case where the CLTV is enforced, splicing brings in additional
>>> >> > subtleties. We should prevent the seller from making any splice-out,
>>> >> > otherwise they would effectively be bypassing the lease CLTV. But we
>>> >> > should allow the seller to splice-in, and the buyer should still be
>>> >> > allowed to splice-in and splice-out. Unfortunately, the seller can
>>> >> > prevent the buyer from splicing funds in and out of the channel, by
>>> >> > simply adding splice-out outputs in any splice attempt: the buyer has
>>> >> > no other choice than aborting that splice.
>>> >> >
>>> >> > I initially thought that we could actually use splice-out to get the
>>> >> > best of both worlds: add a CLTV on the seller's output, but allow them
>>> >> > to splice-out whatever funds are "in excess" of the leased amount. But
>>> >> > that doesn't work, because the buyer can unilaterally reject the splice
>>> >> > attempt, and the seller still ends up with all their funds locked for
>>> >> > the lease duration (even though the seller can also play the abort game
>>> >> > to prevent the buyer from making any splice, as described above).
>>> >> >
>>> >> > Thanks for reading this long post, I think this will be useful if you
>>> >> > are planning on selling liquidity through liquidity ads, as you should
>>> >> > fully understand the drawbacks of the CLTV lock. I do want to highlight
>>> >> > that this is a great protocol, and that a liquidity marketplace is an
>>> >> > important building block for lightning. We'll be working hard to make
>>> >> > sure you can find this in your favorite implementations soon!
>>> >> >
>>> >> > Cheers,
>>> >> > Bastien
>
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